1. Field of the Invention
The present invention relates to optical communications systems. More particularly, the present invention relates to optical amplifiers.
2. Description of the Background Art
A. Optical Communications Systems
Optical communications systems are rapidly becoming a widespread and important technology in telecommunications and networking. Optical communications systems transmit information optically at very high speeds over fiber optics.
The basic components of an optical communications system include: an optical transmitter; fiber optics; and an optical receiver. An optical transmitter incorporates information to be communicated into an optical signal and sends the optical signal. Fiber optics carries the optical signal over a distance. Finally, the optical receiver receives the optical signal and recovers the information therein.
B. Limited Dynamic Range of Optical Receivers
One problem with optical communications systems relates to the limited dynamic range of optical receivers. Optical receivers typically operate properly only within a relatively narrow range of optical signal power levels.
C. Attenuation of the Optical Signal
Another problem in optical communications is attenuation of the optical signal. The attenuation may occur due to transmission of the signal, distribution of the signal, and losses due to insertion of components in the transmission path. Optical amplifiers may be used to compensate for signal attenuation. However, conventional optical amplifiers have had various problems.
i Fiber Amplifiers
One type of conventional optical amplifier is a fiber amplifier. A fiber amplifier includes a length of fiber which is doped so that it may form an active gain medium. Ions of rare-earth metals, such a Erbium, are typically used as the dopant.
The doped fiber is pumped so that a population inversion of electronic carriers occurs in the active medium. The pump typically is an optical source whose wavelength is preferentially absorbed by the ions and yet different from the optical signal to be amplified. The optical signal is input from un-doped fiber to the doped fiber, experiences gain due to stimulated emission as it passes through the doped fiber, and then is output in amplified form from the doped fiber to further un-doped fiber.
One problem with fiber amplifiers is the narrow range of wavelengths that they can amplify when multiple fiber amplifiers are cascaded. Another problem with fiber amplifiers is their transient response to channel drop-out in wavelength division multiplexing systems. Further problems with fiber amplifiers include their relatively large size, slow speed for switching, power inefficiency, difficulties in mass producing them, and their high cost which makes them prohibitively expensive for many applications.
ii. Conventional Semiconductor Optical Amplifiers
Another type of optical amplifier is a conventional semiconductor optical amplifier. Conventional semiconductor optical amplifiers comprise a semiconductor laser like structure which operates below the lasing threshold.
Typically, an electrical current is used to pump the electronic population in the active region of the amplifier. The optical signal is input from fiber optics to the active region of the amplifier, experiences gain due to stimulated emission as it passes through the active region, and is output in amplified form to further fiber optics.
One problem with conventional semiconductor optical amplifiers is that the amount of gain experienced by the input signal is dynamically dependent on the strength of the input signal at a particular instance in time. For instances when the input signal is higher, the signal experiences less gain than for instances when the input signal is lower. This dynamic gain variation is due to gain saturation. Gain saturation occurs when there are insufficient carriers in the conduction band to provide the full amount of gain to the higher signals.
The problems and disadvantages heretofore associated with the prior art are overcome by the present invention. In addition, the gain of the device is made constant and independent of relative signal strength. Furthermore, these problems and disadvantages are overcome while providing a mechanism for adjusting (tuning) the gain of the device.
The present invention comprises a tunable-gain lasing semiconductor optical amplifier. In a preferred embodiment of this invention, the tunable-gain lasing semiconductor optical amplifier comprises a vertical-lasing semiconductor optical amplifier that includes a tunable region which allows the gain of the vertical-lasing semiconductor optical amplifier to be tuned. The tunable region comprises a region whose loss and/or phase may be tuned by adjusting a physical characteristic of the region. For example, the region may comprise a liquid crystal layer whose transmissivity may be adjusted by applying different voltages across the layer, or a cavity mirror whose reflectivity may be adjusted by ion implantation. In an alternative embodiment of this invention, the tunable-gain lasing semiconductor optical amplifier comprises a tunable loss element in series after the gain-clamped semiconductor optical amplifier.